1. Field of the Invention
This invention relates to a semiconductor device, more particularly to a semiconductor device which is especially preferable when applied to a semiconductor device having a hetero-interface therein, such as light receiving elements, e.g., avalanche photodiode (APD) or PIN photodiode, and optical modulators.
2. Description of the Related Art
The APDs, which per se have a multiplication function, are widely used in the field of optical communication because of their wide bandwidth and high gain. In consideration of application of the APDs to high-speed optical communication systems with modulation speeds on the order of 10 Gb/s, however, there is a technical problem to be solved. That is, it is required to overcome the so-called pile-up phenomenon in which the transfer of photo-carriers is impeded at a hetero-interface in a high field application region. As countermeasures against the pile-up, there have been proposed: (1) a method in which a material having a band-discontinuity value between those of two materials constituting the hetero-interface is introduced to the hetero-interface (J. C. Cambell et. al. `InP/InGaAsP/InGaAs avalanche photo diodes with 70 GHz gain band product` Appl. Phys. Lett. 51, 1454 (1987)); (2) a method n which a pseudo-mixed crystal using a thin film is introduced to the hetero-interface (F. Capasso et. al. `Pseudo-quaternary GaInAsP semiconductors: A new GaInAs/InP graded gap superlattice and its applications to avalanche photo diodes` Appl.Phys. Lett. 45(11) p1193 (1984)); and (3) a method in which a semiconductor laminate transition region having an extra thin film barrier layer is provided (Japanese Patent Application Laid-Open (KOKAI) No. 61-224468 (1986)).
The above-mentioned techniques are intended to cope with the pile-up at the hetero-interface in APD or PIN photodiodes, represented by light receiving elements. However, the pile-up phenomenon also considered to cause hindrance of high-speed operations, in many other semiconductor devices such as optical modulators.